This invention relates generally to flow rate indicators, and more particularly to a magnetic follower mechanism for deriving from the movement of the float in a rotameter an external indication that is linearly proportional to flow rate.
In a variable area meter of rotameter for measuring flow rate, only the orifice area is varied as a function of flow, a constant pressure difference being maintained. The typical rotameter is constituted by an upright, tapered tube containing a weighted float which is raised to a position of equilibrium between the downward force of the float and the upward force of the fluid flowing past the float through the surrounding annular orifice. In a rotameter, the flow restriction is the area of the annular orifice, this area being enlarged as the float rises in the tapered tube. The pressure differential is fixed, this being determined by the weight of the float and the buoyant forces.
The term "rotameter" is derived from the fact that the float was originally designed with slots serving to impart a rotational force thereto for the purpose of centering and stabilizing the float. The present trend, however, is toward guided, non-rotating floats. For visual read-out, the vertical position of the float in the variable area tube is indicated along a calibrated scale etched or otherwise formed along the transparent tube surface.
In many flowmeter applications it is necessary to translate the changing vertical position of the float within the flow tube into a corresponding external indication. Where, for example, the process fluid is opaque, the float cannot be seen through the transparent tube, hence direct visual indication along the tube is precluded. In other situations, the flow tube must be made of metal or other opaque material, so that it is necessary to provide means for registering the float position at a point external to the tube.
Mechanical coupling of the float to an external indicator is generally unsatisfactory, since frictional and other loads thereby imposed on the float interfere with its free motion and distort the flow rate reading. In order to minimize loading the float, it is known to use a magnetic, non-mechanical coupling between the movable float and an external indicator.
Thus both the Conkling U.S. Pat. No. 3,315,523 and the Busillo U.S. Pat. No. 3,164,989 disclose arrangements in which the float in the rotameter tube is provided with a guided extension rod to which a bar magnet is attached, the bar magnet moving up and down in accordance with changes in the vertical float position. The bar magnet cooperates with a rotatable follower magnet mounted at one end of a shaft to which an indicating pointer is attached. The follower magnet is caused to rotate in a direction and to an extent which is a function of the bar magnet movement and hence of flow rate.
The difficulty experienced with magnetic followers of the type heretofore known to that when the hydraulic characteristics of the rotameter are aligned to the follower output by small manipulations of zero and span adjustment, the movement of the float from its minimum to its maximum position is accompanied by an indicator movement having a bowed characteristic that is essentially symmetrical with respect to the 50% flow rate position. In other words, though the lift of the float in the tube has a straight-line relationship to the indication, the float lift exhibits a non-linear or curved relationship to flow rate which takes the form of a bow-shaped curve extending between the extremities of float travel.
In order, therefore, for the indicator to provide an accurate reading of flow rate, one must provide a scale having a non-linear calibration which takes into account the bowed characteristic. While a non-linear scale is generally acceptable in a simple, low-cost flowmeter, it is not satisfactory for indicating transmitters that must produce a linear output reading directly proportional to flow rate.
To overcome this drawback, it has been known to use bias magnets in conjunction with the magnetic follower to compensate for the bowed characteristic thereof, or to use linkages and tailored parts to linearize the follower motion as a function of flow rate. The objection to each of these expedients is that in adjusting the follower to render it linear, this adjustment also affects zero and span, so that it then becomes necessary to readjust zero and span and this in turn throws off the linearity adjustment. In practice, therefore, a series of rather delicate adjustments becomes necessary in order to attain the desired values of zero and span as well as acceptable linearity.